Metal bellows are used for the hermetically sealed isolation of movements. For reasons of better formability and corrosion resistance, they usually consist of austenitic high-grade steels. The usual requirements for bellows concern the mobility, service life, spring stiffness and pressure resistance of the bellows. A movement or a combination of movements of a defined magnitude with a fixed minimal service life, i.e., with a certain number of load cycles, shall often be endured.
An elementary design criterion for metal bellows is the magnitude of the movements absorbed. This movement magnitude has direct effects on the required installation space of the corrugated bellows. Often, a large movement with a high number of load cycles with low installation space requirement shall be achieved.
A high corrugation height and a high number of bellows corrugations with as low a wall thickness as possible are advantageous for a high absorption of movements. The low wall thickness has an advantageous effect on the absorption of movements, provided the load of the metal bellows mostly takes place in a path-controlled manner, i.e., the absorption of movements is determined by the stimulating system. The absorbed movements are expressed as deflections in the bellows corrugations. The greater the wall thickness of the bellows is, the greater are the bending stresses generated.
However, the pressure resistance of the bellows also drops with decreasing wall thickness. Should a bellows have a pressure-resistant design, then this is limited in its mobility and its service life due to the necessary great wall thickness in a given installation space.
Finally, an as low as possible spring stiffness of the bellows is often required as well. As also for the service life, a small wall thickness of the bellows is decisive for a low spring stiffness of the bellows as well.
The often used austenitic high-grade steels, e.g., 1.4301, 1.4401, 1.4404, 1.4509, 1.4541, 1.4571, 1.4828 or nickel-based alloys, such as 2.4600, 2.4816 or 2.4856 and duplex steels, such as 1.4362 or 1.4462 are characterized by their excellent formability. On the other hand, their strength lies far below the strength that can be achieved with ferritic steels. The use of bellows made of austenitic high-grade steels is especially limited in terms of their compressive strength. If a high compressive strength shall be achieved, the wall thickness of the bellows must be high. This leads to marked drawbacks in the fatigue strength and stiffness of the bellows.
If very high pressure resistances shall be achieved, another problem arises: Metal bellows are usually manufactured by hydraulic forming by means of internal pressure. A pressure, which brings the starting material into the final state, is also suitable for deforming the finished product again later. That is, the pressure used for forming must always be greater than the later pressure, to which the finished bellows is exposed during operation.
For some applications, e.g., in injection nozzles, the required operating pressures are, however, very high. Modern diesel injection systems are designed for operating pressures of above 2,000 bar. Such pressures cannot up to now be achieved in bellows manufacturing.
To increase the pressure resistance of bellows, metal bellows made of a hardenable, usually ferritic steel are already manufactured according to the state of the art and then hardened and tempered after the manufacture. One drawback in the use of ferritic steels is the low elongation after fracture compared to austenitic steels. As a result of this, only a low corrugation height can be achieved at the bellows. A further exclusion criterion is the corrosion resistance required in many applications, which makes the use of austenitic steels necessary.
One possibility for increasing pressure resistance of bellows while maintaining controllable forming pressures shown in EP 1 985 388 B1 is the use of a material with two metastable states, one of which is soft and ductile, the other state is very hard and has high strength. The change in states for this is usually achieved by a special heat treatment. The steels used for this are, however, expensive to procure and process. In addition, the soft, ductile state is only present at high temperatures, which considerably complicates the forming process and makes it more expensive.
A further difficulty lies in the connection of the thus manufactured parts to the surrounding area. On the one hand, some of the kinds of steel described in EP 1 985 388 B1 cannot be welded easily or not at all; on the other hand, the metastable states achieved after the forming are carried out first after the welding of the bellows; this may lead to thermal distortion and impairment of the corrosion resistance at the connection parts.
An especially pressure-resistant bellows geometry with a corrugation height that is lower than the corrugation length is described in the cited publication in connection with the hardenable materials. However, such a geometry is highly unfavorable for the absorption of movements of the bellows. A comparatively long bellows and large installation space requirements resulting therefrom must be used here to achieve acceptable service lives in a given movement requirement. It is known to increase the surface quality of a metallic material by incorporating carbon and/or nitrogen atoms in a very thin surface layer of up to 35 μm in order to achieve especially an improved wear and scratch resistance.